A presently used method of detecting rail flaws is ultrasonic. A quartz crystal is electronically pulsed to generate a burst of oscillations, which signal is applied down through and perpendicular to the top of a rail toward the base from a vehicle passing over the rail rapidly (see U.S. Pat. No. 4,137,776 of the same inventors entitled "Automatic Base Gate Positioning Circuit," issued on Feb. 6, 1979). If a flaw, such as a crack or split, is encountered within the rail, part of the energy of the burst is reflected, with direction of reflection a function of incident angle. The reflected burst is called an echo and is detected through the top of the rail, amplified and subjected to various levels of electronic discrimination. Within the present signal processing systems discrimination consists of a threshold, a D.C. level manually adjusted and set such that background noise is optimally suppressed, which the signal must exceed; and manually set time gates, which when in coincidence with the return signal, determine the region of the rail in which the flaw is located, typically web and base of the rail.
If no flaw is encountered, then reflection of the transmitted signal will occur from the bottom of the rail which is called the base.
In normal operation for a given rail type, using a video screen display of the return amplified and processed signal, the time gates are set and then the base echo signal is adjusted to a predetermined percentage of the video screen height by varying the gain of the active amplifier, assuming all other variables affecting optimization have been properly adjusted. The threshold level is set to exclude noise.
With the equipment set up as described, the test vehicle, which contains the entire flaw detection system, proceeds along the rail. Since the ultrasonic bursts are generated at some distance driven rate, typically 10 bursts/inch of rail, a depth profile along the rail is obtained.
As the detection system moves from rail to rail, however, the base echo reflection in flawless rail may vary in strength primarily because the differing granular structure of various rail will cause more or less attentuation. Another cause of attenuation is top surface contamination of the rail resulting in less efficient transfer of ultrasonic energy into and out of the rail, thereby causing return signal attenuation. Under these circumstances if the echo signal amplitude is altered then a flaw reflection in the same rail will be altered by the same amount. As a result flaws can go undetected or overdetected. The obvious solution is to change the gain of the amplifier to counteract the base echo amplitude variation. In the prior art this is done manually. Since manual response is inherently slow an electronic automatic gain control is needed.
However, some technique must be incorporated within any automatic gain control which takes into account a large number of consecutive base echoes and performs some averaging function on them in order not to be affected by rail flaws or normal rail characteristics such as bolt holes or rail ends.